1. Field of the Invention
This invention relates to light emitting diode packages and displays utilizing light emitting diode packages as their light source.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Technological advances over the last decade or more has resulted in LEDs having a smaller footprint, increased emitting efficiency, and reduced cost. LEDs also have an increased operation lifetime compared to other emitters. For example, the operational lifetime of an LED can be over 50,000 hours, while the operational lifetime of incandescent bulb is approximately 2,000 hours. LEDs can also be more robust than other light sources and can consume less power. For these and other reasons, LEDs are becoming more popular and are now being used in more and more applications that have traditionally been the realm of incandescent, fluorescent, halogen and other emitters.
In order to use an LED chip in conventional applications it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical two-pin LED package/component 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup 13 may be filled with an encapsulant material 16 and a wavelength conversion material, such as a phosphor, can be included over the LED chip or in the encapsulant. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly can then be encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to direct or shape the light emitted from the LED chip 12.
FIG. 2 shows a top view of a conventional LED package 20 similar to the package 10 shown in FIG. 1 and including an LED chip 22 mounted at the base of a reflective cup 24. Wire bonds 26a and 26b are included to connect to the ohmic contacts of the LED chip 22, and the reflective cup 24 is filed with an encapsulant material 28. In package 20, the reflective cup 24 is oval shaped and the LED chip 22 is rectangular shaped, with the LED chip 22 and reflective cup 24 being longitudinally aligned. That is, longer edges of the LED chip are aligned with the reflective cup axis running along the elongated direction of the reflective cup.
Different LEDs packages, such as those shown in FIGS. 1 and 2, can be used as the light source for displays, both big and small. Large screen LED based displays (often referred to as giant screens) are becoming more common in many indoor and outdoor locations, such as at sporting arenas, race tracks, concerts and in large public areas such as Times Square in New York City. With current technology, some of these displays or screens can be as large as 60 feet tall and 60 feet wide. As technology advances it is expected that larger screens will be developed.
These screens can comprise thousands of “pixels” or “pixel modules”, each of which can contain a plurality of LEDs. The pixel modules can use high efficiency and high brightness LEDs that allow the displays to be visible from relatively far away, even in the daytime when subject to sunlight. The pixel modules can have as few as three or four LEDs (one red, one green, and one blue) that allow the pixel to emit many different colors of light from combinations of red, green and/or blue light. In the largest jumbo screens, each pixel module can have dozens of LEDs. The pixel modules are arranged in a rectangular grid. In one type of display, the grid can be 640 modules wide and 480 modules high, with the size of the screen being dependent upon the actual size of the pixel modules.
Most conventional LED based displays are controlled by a computer system that accepts an incoming signal (e.g. TV signal) and based on the particular color needed at the pixel module to form the overall display image, the computer system determines which LED in each of the pixel modules is to emit light and how brightly. A power system can also be included that provides power to each of the pixel modules and the power to each of the LEDs can be modulated so that it emits at the desired brightness. Conductors are provided to apply the appropriate power signal to each of the LEDs in the pixel modules.
LED displays are rarely mounted at the viewer's eye level, and are more typically mounted at an elevation above eye level, such as on the side of a building or the top of the grandstands in a stadium. Referring now to FIG. 3, a conventional LED display 30 is shown mounted at an elevated point above the eye level of the viewer 32. The viewer 32 is typically positioned below the display 30 and looks up to the display such that the viewer's line of sight 34 to the display 30 is at an angle θ to the display's perpendicular emission direction 36. The LED display in FIG. 3 typically comprises a plurality of emitters 38 such as those shown in FIGS. 1 and 2 that exhibit a peak emission that is near the center of the horizontal and vertical axis.
Having a display comprising a plurality of LED packages 38 can result in display peak emission characteristics emitting in the perpendicular direction 36, as shown. The Iv and far field pattern (FFP) peak emission characteristics for the LED display 30 can be perpendicular to the display along the perpendicular axis 36. The viewer's line of sight 34 is below perpendicular when the display 30 is mounted at an elevated point; much of the light emitted by the display is not seen by the viewer and is wasted. This can be true for viewers below the display and the side of the display. One way to reduce the amount of light that is wasted is by mounting the display at an angle to better match the viewer's line of sight 34, but this can require complex and expensive mounting hardware that is difficult to use, particularly for very large displays mounted at high elevations.
Viewers are often not directly in front of an LED based display when it is viewed. Depending on where the viewer is located the horizontal viewing angle can be different. Furthermore, when a person is moving by an LED display, such as walking by, it is viewed at many different horizontal angles. Typical LED displays with peak emissions near the center can experience a drop-off in emission intensity at different horizontal angles. The far field pattern (FFP) for the different LED packages in each of the pixels can also be different such that the LED display can experience image quality variations when viewed from different angles.
Because of this, it can be important for the FFP emission characteristics of the red, green and blue LED packages used in LED displays pixels to be smooth, as wide as possible, and matched between the red, green, and blue colors. Standard geometry LED packages as shown in FIGS. 1 and 2 allow for the longest reflection surface of the reflector cup (wide viewing angle) to be parallel with the longest emission side of the chip (wide viewing angle). This geometry also allows the shortest reflection surface of the reflector cup (narrow emitting angle) to be parallel with the shortest emission side of the chip (narrow viewing angle). This geometry also minimizes bond wire length, which can help minimize the wire bond failure rate. The challenge with this arrangement is that it can require a near perfect match between the LED chip far-field pattern, the reflective cope and LED dome. Without this perfect match a large amount of diffuser can be necessary in the dome or encapsulant. Diffusers, however, can absorb light emitting from the packages, and thereby can reduce the emission brightness.